Mattress supporting spring frames in the form of four corner connected angle irons are old and well-known. They are e.g. used universally in childs' cribs. The design of such a frame is relatively simple--the end portions of two side and two end angle irons are riveted together to form a rectangular open frame that mounts the spring fabric. Angle irons are used because they are inexpensive and available, and the mattress may find support at its edges and head and foot on the horizontal flanges of the angle irons, while the vertical flanges stiffen the angle irons against deflection under weight of the mattress and the occupant. Improvement is always looked for, in expense as well as safety and comfort, and the present invention provides rounded corners (safer), eliminates all rivets, and aids in easier, faster, and cheaper handling and assembly.